A process operator may wish to monitor any of a number of different measurands within piping arrangements such as the pressure, temperature, flow velocity, flow velocity, density and acoustic flow or process equipment operating noise. In the field of oil and gas this is often achieved by installing a plurality of instruments into a section of the piping referred to as the ‘instrument hub spool piece’ or ‘instrument header’. Each instrument is fitted to a separate port and is placed in contact with process fluid contained within and monitors a respective parameter from it. However, it can be difficult and costly to accommodate multiple different instruments at the same cross sectional area of piping or in close proximity to process flow control equipment due to the need for multiple ports, each one with threaded or flanged connection mechanisms.
One example of where it may be desirable to monitor fluids is in order to detect the presence of leaks in piping or other process equipment. Distributed Temperature Sensing (DTS) and Distributed Acoustic Sensing (DAS) are two techniques that have been used for this purpose. In DTS, a fiber-optic cable is laid along the outside of a pipeline, or potentially in an outer annulus of the pipeline, separate from the fluid flow. A light source and a receiver are coupled to one end of the fiber optic cable. When a fluid leaks from the pipeline it comes into contact with a section of the cable and may change the temperature of this section. This temperature change can be detected by injecting light into the fiber and monitoring the spectral response from any light that is scattered back along the cable to the receiver. DTS only works however if the leaking fluid is at a different temperature than the cable. DAS works in a similar fashion however in this case it is the vibrations caused by a leaking fluid which affect the spectral response of the scattered light. By laying an optical fiber along the outside of the pipeline it is also possible to obtain pressure, temperature and strain readings from the local environment surrounding the pipeline. This may give an indication of the conditions affecting the outside of the pipeline, but is not necessarily indicative of the conditions inside the pipeline.
Additionally, neither DTS nor DAS may be sensitive enough to detect the smallest leaks. There therefore remains a need for improved methods and apparatus for monitoring fluids.